Over the past few years, good progress has been made in understanding oxidative stress and its consequences for macromolecules such as proteins. Oxidatively modified proteins have been shown to accumulate during aging, cancer and many other pathological conditions. This has further increased the interest in a stress situation that is simply unavoidable for organisms that live an anaerobic lifestyle. Now, a number of proteins have been identified which use oxidative stress as a regulator for their functional activity. These proteins have in common highly reactive cysteines that sense the changes in the redox potential of the environment and translate them into changes of protein conformation and activity. We discovered that one of these novel redox regulated proteins is the molecular chaperone Hsp33. Hsp33 is a folding-helper protein that appears to be specialized to protect cells against the otherwise lethal effects of oxidative stress. We will now analyze how Hsp33 performs this task. We will identify the conformational rearrangements that accompany the activation and inactivation process of Hsp33 and we will determine the underlying mechanism of substrate recognition. We will define what Hsp33's substrates are under heat and oxidative stress conditions. This will not only allow us to identify and classify oxidation sensitive proteins which are protected by Hsp33 but may also explain how Hsp33's substrate specificity differs from that of other molecular chaperones. We will explore the possibility that Hsp33 presents some of its substrate proteins to the proteolytic system of the cell. It is known that many oxidatively modified proteins are irreversibly damaged and need to be degraded before they can accumulate as harmful, insoluble protein aggregates in the cell. Since Hsp33 has a very high affinity to protein folding intermediates, this substrate transfer to cellular proteases might also explain why Hsp33's chaperone activity needs to be down regulated under non stress conditions. Otherwise, Hsp33 has the potential of interfering with ,the normal protein folding process by prematurely presenting folding proteins to the proteolytic system of the cell.